If you are referring to the little triangular bump on the lower edge, That is the polarizing device. JR & Spektrum servo leads have two corners beveled at a 45 degree, it is slight but it is there. Full size JR receivers, many times have that little bump along both sides of the plug area. Works pretty good, but you can force them in wrong.

If you have Futaba servos, they will have a ridge along one edge of the plug, I just trim that off, scrape a bevel on the edges and plug them in.

I have never found a need to cut the receiver case.

In your picture, the signal wire will be the top row, then positive, with negative on the bottom row. The signal wire is almost always the pin that would have the closest relationship to the circuitry on the board. Pos & neg are common from pin to pin, or left to right